Sandra I. Davis
Objectives—The student will:
-
-state the overall function of the endocrine system
-
-define endocrine
-
-state the difference between endocrine and exocrine gland
-
-state the two types of hormones
-
-define peptide and steroid hormone
-
-state what a target or receptor site is
-
-state research techniques in hormone discovery and function.
The endocrine system is composed of cells that are organized into endocrine and exocrine glands. Endocrine glands secrete hormones into the bloodstream which in turn circulate throughout the body to specific locations at which they bond to receptors. (see diagram 1). Exocrine glands do not secrete hormones.
Hormones are like heat seeking missiles in that once fired, they will continue to seek out it’s target until contact. Once contact has been made, the missile carries out it’s mission, and a reaction takes place. When a hormone reaches its receptor site it also initiates a reaction within the cell or organ. Hormones are chemical substances made by the gland from which it is secreted which influence the activities of the cells, tissues, and organs of the body.
Hormones are divided into two groups based on their biochemical structure called peptides and steroids. Peptide hormones are made up of amino acids which are strung together they are composed of as few as 3 and as many as 237 amino acids. Peptides are synthesized in the pituitary, parathyroids, and pancreas. Examples of peptide hormones are oxytocin, vasopressin, insulin, thyrotropin, follicle-stimulating hormone, (FSH), and luteinizing hormone, (LH) The cells which make these hormones contain the genetic code which specifies the amino acid sequence for each hormone. Peptide hormones bind to their receptors on the surface of the cell.
Steroid hormones are made from cholesterol which forms a characteristic four ring structure. Steroid hormones bond to their receptors inside the cell membrane, They play an important sole in carbohydrate metabolism and electrolyte balance. Steroids are synthesized in the adrenal glands, ovaries, and testes.(see diagram 1)
Each gland produces one or more hormones, and it’s activity is regulated so that the proper amount’ of hormone is released at the proper time. When certain hormones reach their target sites the gland sends another hormone or nerve signal back to the secreting gland and signals it to stop making that specific hormone. (see diagram 3). This system can be compared to checks and balances. The endocrine system and nervous system work together ta make ‘sure that there is not an overload of hormone production.
The inquiry into the function of the human body began thousands of years ago with the Greeks, and their theory that all things were composed of air, fire, water, and earth. Accurate knowledge of the function of the endocrine glands came about through the observation of diseased glands, removal of the organ, injecting organ extracts into the body, and through isolation of the hormone extracts themselves during which it’s chemical makeup was determined. In the mid-l9th century Addison, Bertholt, and Brown-Sequard conducted the first experimental studies in endocrinology. Addison noted a condition in which a variety of signs and symptoms were associated with the destruction of the adrenal gland by disease. The symptoms include hypoglycemia, which leads to muscular weakness , mental lethargy, and weight loss, which occur as a result of lowered plasma sodium concentration.
In 1949 Bertholt transplanted testes into a previously castrated cockerel and found that it prevented the retrogressive changes in sexual maturation and behavior. As a result of this, he concluded that the effects of the transplanted testes might be due to some substance that was secreted into the blood. Another technique is autoradiography in which a hormone containing radioactive atoms is infected into the animal and ‘later d picture is taken to find where the hormones are located. Other techniques include cross-circulation, and parabiosis in which a permanent union is established between two young animals and cross circulation develops as the tissue joined through surgery heals. This technique has been useful ln establishing the relationship between the pituitary and the gonads.
Classroom Activities
-
1. On diagram one, discuss with the students the route that the hormones will follow once excreted from the gland. Ask questions such as,’Where do the hormones go when excreted from the gland?” Use a diagram of the body to show the circulatory system and follow it to various parts of the body that the hormone may go to.
-
2. Hand out a blank chart with peptide on one side and steroid on the other. Have the students list the differences between each of hormone. Use diagram 1 as a reference,
-
3. Hand out a blank diagram of a cell membrane. Have the class draw how peptide hormones bind on the surface, and steroid hormones bind inside the cell membrane, using diagram 1 as a reference,
-
4. Have the class fill in a blank chart of the hormonal checks and balances.
(figure available in print form)
DIAGRAM 1
(figure available in print form)
DIAGRAM 2
(figure available in print form)
DIAGRAM 3
